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3. Evidence as a Guide to Truth: Evidence as Sign, Symptom, or Mark The situation in which I would properly be said to have evidence for the statement that some animal is a pig is that, for example, in which the beast itself is not actually on view, but I can see plenty of pig-like marks on the ground outside its retreat. If I find a few buckets of pig-food, that's a bit more evidence, and the noises and the smell may provide better evidence still. But if the animal then emerges and stands there plainly in view, there is no longer any question of collecting evidence; its coming into view doesn't provide me with more evidence that it's a pig, I can now just see that it is. —J.L. Austin, Sense and Sensibilia‘Not enough evidence God! Not enough evidence!’ —Bertrand Russell, upon being asked what he would reply if, after dying, he were brought into the presence of God and asked why he had not been a believer If E is evidence for some hypothesis H, then E makes it more likely that H is true: in such circumstances, E confirms H. On the other hand, if E is evidence against H, then E makes it less likely that H is true: E disconfirms H. Verification is the limiting case of confirmation: a piece of evidence verifies a hypothesis in this sense just in case it conclusively establishes that hypothesis as true. At the other end of the spectrum, falsification is the limiting case of disconfirmation: a piece of evidence falsifies a hypothesis just in case it conclusively establishes that the hypothesis is false. It is at least somewhat controversial whether full-fledged verification or falsification in this sense ever occurs.http://plato.stanford.edu/entries/evidence/notes.html#16 16 Plausibly, there are some propositions whose truth or falsity we grasp in an utterly direct, unmediated way. Consider, for example, simple arithmetical truths such as the proposition that 2+2=4. Traditionally, such truths have been held to be ‘self-evident’; allegedly, they need only to be understood in order to be known. If the truth value of every proposition were transparent in this way, perhaps we would have little or no need for evidence. In contrast, a central function of evidence is to make evident that which would not be so in its absence. In general, we rely on evidence in cases in which our access to truth would otherwise be problematic. One's recognition that the earth is roughly spherical in shape seems to depend on one's evidence in a way that one's recognition that 2+2=4 does not. Of course, it can be a contested matter whether one's access to truth in some domain is problematic—and thus, whether one is dependent upon evidence for grasping truths about that domain. Common sense holds that we often have unproblematic access to facts about our immediate physical environment via sense perception; perhaps in part for this reason, common sense regards it as at the very least odd, if not simply wrong, to say that one who finds himself face-to-face with what is clearly a pig thereby has strong evidencethat the animal is a pig. (Although it would no doubt also be odd to assert that one lacks evidence that the animal is a pig in such circumstances.) In contrast, much traditional epistemology holds that one's access to such truths is always deeply problematic; what is unproblematic, rather, is one's recognition that one's experiences represent the world as being a certain way. Hence, much traditional epistemology construes the relationship between one's experiences and one's beliefs about the physical world on the model of the relationship between evidence and hypothesis. On this model, the fallibility of sense perception is assimilated to the fallibility of non-deductive inference. (The above quotation from Austin is, of course, a protest against the model in question.) As a general matter, evidence seems to play a mediating role vis-a-vis our efforts to arrive at an accurate picture of the world: we seek to believe what is true by means of holding beliefs that are well-supported by the evidence, and we seek to avoid believing what is false by means of not holding beliefs that are not well-supported by the evidence. The picture is well summarized by Blanshard: ‘Surely the only possible rule’, one may say, ‘is to believe what is true and disbelieve what is false.’ And of course that would be the rule if we were in a position to know what was true and what false. But the whole difficulty arises from the fact that we do not and often cannot. What is to guide us then?… The ideal is believe no more, but also no less, than what the evidence warrants (1974: 410–411). Indeed, it is plausible to suppose that both the capacity of evidence to justify belief (Section 1) and the fact that rational thinkers respect their evidence (Section 2) depends upon the connection between evidence and truth. Why should attending to evidence constitute a promising way of pursuing an accurate view of the world? This question is more readily answered on some conceptions of evidence than on others. Thus, consider a theory according to which evidence consists of facts. Given that no true proposition is inconsistent with any fact, one has an immediate rationale for not believing any proposition that is inconsistent with one's evidence, for only propositions that are consistent with one's evidence are even candidates for being true. The same holds for Williamson's conception of evidence as knowledge: inasmuch as any known proposition is true, inconsistency with one's evidence entails inconsistency with some truth. Put the other way around: if evidence consists of facts or known propositions, then no body of evidence rules out any truth. Notice that the same is not true for conceptions of evidence according to which one's evidence consists of one's beliefs, or one's experiences, or propositions of which one is psychologically certain: that a proposition is inconsistent with one of my beliefs, or with the content of one of my experiences, or with a proposition of which I am psychologically certain, does not guarantee that it is false. Perhaps the root notion of evidence is that of something which serves as a reliable sign, symptom, or mark of that which it is evidence of. In Ian Hacking's phrase, this is ‘the evidence of one thing that points beyond itself’ (1975: 37). Thus, smoke is evidence of fire, Koplik spots evidence of measles, a certain distinctive and off-putting smell evidence of rotten egg. Here, the paradigm would seem to be that of straightforward correlation: the reason why smoke counts as evidence of fire, but not of impending rain, is that smoke is a reliable indicator or symptom of the former but not of the latter. Taken at face value, the idea of evidence as reliable indicator tends to encourage an inclusive picture of what sorts of things are eligible to count as evidence, according to which either mental or non-mental objects, events and states of affairs can qualify as such. For such entities would seem to be perfectly capable of standing in the relevant relation to other objects, events and states of affairs. Consider the claim that (1) Koplik spots are evidence of measles. On what is perhaps its most natural reading, the truth of this claim was an empirical discovery of medical science. At a certain point in time, it was discovered that Koplik spots are a reliable indication of measles—something which was true, presumably, long before the discovery in question. Here, the evidence relation is understood as a relation that either obtains or fails to obtain independently of what anyone knows or believes about its obtaining.http://plato.stanford.edu/entries/evidence/notes.html#17 17 To the extent that one is concerned to arrive at an accurate picture of the world, knowledge of instances of this relation—roughly, knowledge of what bits of the world tend to accompany what other bits of the world—would seem to be exactly the sort of thing which one is seeking. When the evidence relation is construed in this way, investigating it is of a piece with investigating the world itself. Similarly (2) Smoke is evidence of fire would seem to have the same empirical status as (1), differing chiefly in that it is much more widely known. When evidence is understood in this way, it is no mystery why attending to evidence is a good strategy for one who is concerned to arrive at an accurate picture of the world: given that Koplik spots are in fact a reliable indicator of measles, it obviously behooves those who are concerned to have true beliefs about which individuals are suffering from measles to pay attention to facts about which individuals have Koplik spots. Similarly, given that smoke is in fact a reliable indicator of fire, those who are concerned to have true beliefs about the presence or absence of fire do well to pay attention to the presence or absence of smoke. Thus, when we understand ‘E is evidence for H’ as more or less synonymous with ‘E is a reliable indicator of H’, the connection between evidence and truth seems easily secured and relatively straightforward. Of course, although the presence of Koplik spots is in fact a reliable guide to the presence of measles, one who is ignorant of this fact is not in a position to conclude that a given patient has measles, even if he or she is aware that the patient has Koplik spots. Someone who knows that Koplik spots are evidence of measles is in a position to diagnose patients in a way that someone who is ignorant of that fact is not. In general, the extent to which one is in a position to gain new information on the basis of particular pieces of evidence typically depends upon one's background knowledge. This fact is a commonplace among philosophers of science and has also been emphasized by philosophically-sophisticated historians.http://plato.stanford.edu/entries/evidence/notes.html#18 18 Suppose that one knows that a particular patient has Koplik spots but is ignorant of the connection between Koplik spots and measles. Moreover, suppose that one's ignorance is not itself the result of any prior irrationality or unreasonableness on one's part: rather, one has simply never had the opportunity to learn about the connection between Koplik spots and measles. In these circumstances, does one have evidence that the patient has measles? Taken in one sense, this question should be answered affirmatively: one does have evidence that the patient has measles, although one is not in a position to recognize that one does. However, the idea that one has evidence in such circumstances seems to sit somewhat awkwardly with the themes that evidence tends to justify belief, and that rational thinkers are sensitive to their evidence. For consider the moment when one first learns that the patient has Koplik spots. Given one's ignorance of the connection between Koplik spots and measles, one is not in any way unreasonable if one fails to become more confident that the patient has measles. Indeed, given one's ignorance, it seems that one would be unreasonable if one didbecome more confident that the patient has measles upon learning that she has Koplik spots, and that a belief that the patient has measles held on this basis would be unjustified. This suggests that the notion of evidence in play in statements such as ‘evidence tends to justify belief’ and ‘rational thinkers respect their evidence’ cannot simply be identified with evidence in the sense of reliable indicator. Let's call evidence in the former sense normative evidence, and evidence in the latter sense indicator evidence. Although the normative notion of evidence cannot simply be identified with the indicator notion, we would expect the two to be closely linked, inasmuch as one's possession of normative evidence frequently depends upon one's awareness that one thing is indicator evidence of something else. Reflection on the role that considerations of background theory play in determining how it is reasonable for one to respond to new information have convinced some that the normative notion of evidence is better understood in terms of a three place-relation rather than a two-place relation. According to this view, judgements of the form ‘E is evidence for H’—when this is understood as more or less synonymous with ‘E tends to make it more reasonable to believe H’— are typically elliptical for judgements of the form ‘E is evidence for H relative to background theory T’. Thus, given that your background theory includes the claim that Koplik spots are a reliable indication of measles, the fact that a particular patient has Koplik spots constitutes normative evidence for you (gives you a reason to believe that) the patient has measles. On the other hand, given that my background theory does not include the claim that Koplik spots are a reliable indication of measles, the fact that the same patient has Koplik spots does not constitute normative evidence for me (give me a reason to believe that) the patient has measles. The view that the status of E as normative evidence for H can depend upon considerations of background theory immediately raises questions about the status of the background theory itself. Given that one's background theory consists of some set of propositions, which set is it? Is it the set of propositions that one knows? Or rather, the set of propositions that one believes? Or perhaps, the set of propositions that one justifiably believes? It seems that E might count as evidence for Hrelative to the set of propositions that one believes but not relative to the set of propositions that one knows (or vice versa)—which of these, if either, determines whether E is evidence for H, in the sense that one's possession of E tends to justify one in believing that H is true? The issues that arise here are subtle and delicate; Christensen (1997) is a careful and illuminating discussion. A note about confirmation theory. Although philosophy had in some sense long been concerned with questions about when evidence makes a theory more likely to be true, the investigation of this relationship reached new levels of systematicity and rigor during the positivist era. The positivists thought of philosophy as ‘the logic of science’; they thus took it to be a central task of philosophy to furnish detailed analyses and explications of fundamental scientific concepts such as explanation and confirmation. Hempel (1945) and Carnap (1950) each distinguished two different ‘concepts’ of confirmation: the ‘classificatory’ or ‘qualitative’ concept on the one hand and the ‘quantitative’ concept on the other. Roughly, the classificatory concept is employed in the making of yes-or-no judgements about whether a given piece of evidence does or does not support a given hypothesis. Thus, it is the classificatory concept which is in play when one is concerned with judgements of the following form: ‘Hypothesis H is confirmed by evidence E’. On the other hand, the quantitative concept is employed in making numerical judgements about how much support a hypothesis derives from a given piece of evidence (e.g,. ‘Hypothesis H is confirmed by evidence E to degree R’).http://plato.stanford.edu/entries/evidence/notes.html#19 19 Formal theories attempting to explicate each of these notions were developed. Hempel (1945) took the lead in attempting to explicate the qualitative concept while Carnap (1950, 1952) concentrated on the quantitative concept. During this period, the philosophical study of the relationship between evidence and theory took on, perhaps for the first time, the characteristics of something like normal science, and became a discipline replete with technical problems, puzzles, and paradoxes, the anticipated solutions to which were viewed as items on the agenda for future research.http://plato.stanford.edu/entries/evidence/notes.html#20 20 Here lie the origins of present-day confirmation theory, as represented by Bayesianism in its protean forms (See, e.g., Jeffrey 1965, 1992, 2004, Horwich 1983, Howson and Urbach 1993) and its rivals (e.g., Glymour's (1980) ‘bootstrapping’ model of confirmation). Although Carnap's own vision for confirmation theory was ultimately abandoned,http://plato.stanford.edu/entries/evidence/notes.html#21 21 the quantitative approach that he championed proved influential to the subsequent development of the subject. In particular, the emphasis on attempting to understand confirmation in quantitative terms paved the way for the increased use of mathematics—and specifically, the probability calculus—in the philosophical study of evidence. The idea that the probability calculus provides the key to understanding the relation of confirmation is central to Bayesianism, the dominant view within contemporary confirmation theory. An examination of Bayesianism will not be undertaken here.http://plato.stanford.edu/entries/evidence/notes.html#22 22Instead, we will simply take note of the explication of the concept of evidence which the Bayesian offers. At the outset of the present section, we noted that evidence confirms a theory just in case that evidence makes the theory more likely to be true; evidence disconfirms a theory just in case the evidence renders the theory less likely to be true. The Bayesian takes these platitudes at face value and offers the following probabilistic explication of what it is for E to be evidence for H: E is evidence for H if and only if Prob(H/E) > Prob(H).http://plato.stanford.edu/entries/evidence/notes.html#23 23 That is, E is evidence for H just in case the conditional probability of H on E is greater than the unconditional probability of H. Thus, the fact that the suspect's blood is on the knife is evidence for the hypothesis that the suspect committed the murder if and only if the probability that the suspect committed the murder is greater given that his blood is on the knife than it would be otherwise. Similarly E is evidence against H if and only if Prob(H/E) < Prob(H). That is, E is evidence against H just in case the conditional probability of H on E is less than the unconditional probability of H. Thus, the fact that the suspect's fingerprints are not on the knife is evidence against the hypothesis that the suspect committed the murder if and only if the probability that the suspect committed the murder is lower given the absence of his fingerprints on the knife than it would be otherwise. Within this probabilistic model, verification (in the sense of conclusive confirmation) would involve bestowing probability 1 on an hypothesis while falsification would involve bestowing probability 0 on it. This straightforward probabilistic model of evidence and confirmation is an attractive and natural one. Indeed, hints of it are found in Anglo-American law.http://plato.stanford.edu/entries/evidence/notes.html#24 24 The model is not without its critics, however. Achinstein (1983) contends that something can increase the probability of a claim without providing evidence for that claim. For example, the information that seven-time Olympic swimming champion Mark Spitz went swimming increases the probability that Mark Spitz has just drowned; nevertheless, according to Achinstein, the former hardly constitutes evidence that the latter is true.http://plato.stanford.edu/entries/evidence/notes.html#25 25According to a line of thought in Goodman (1955), the notion of confirmation that is crucial for science is not to be understood in terms of straightforward increase-in-probability. Thus, consider the generalization that All of the change in my pocket consists of nickels. Examining one of the coins in my pocket and finding that it is a nickel undoubtedly increases the probability that the generalization is true, inasmuch as it now has one less potential falsifier. But Goodman contends the relevant observation does not confirm the generalization, inasmuch as the observation should not make one more confident that any of the other, as-yet-unexamined coins in my pocket is itself a nickel. (According to Goodman, although ‘accidental generalizations’ such as ‘All the coins in my pockets are nickels’ can have their probabilities raised by observations, they cannot be confirmed by them, in the way that ‘law-like generalizations’ can be.) In general, the idea that the probability calculus provides the key to understanding the concept of evidence has found greater favor among philosophers of science than among traditional epistemologists. In the next and final section, we turn to a cluster of themes that have also been much emphasized by philosophers of science, themes which came to the fore as a result of philosophical reflection upon the role that evidence plays within scientific practice itself.